In order to improve the creep strength of the heat resistant steels at elevated temperatures over 700°C, a new attempt has been demonstrated using carbon and nitrogen free Fe-Ni martensitic and austenitic alloys strengthened by Laves phase such as Fe2W and Fe2Mo. It is important that these alloys are independent of any carbides and any carbo-nitrides as strengthening factors. The high temperature creep tests over 700°C exceed 36,000 hours and the test is continued. Creep behavior of alloys is found to be completely different from that of the conventional high-Cr ferritic steels. The alloys exhibit gradual change in the creep rate with strain both in the transient and acceleration creep regions, and give a larger strain for the minimum creep rate. Effect of Cr on the Fe-12Ni-9Co-10W alloys on the creep properties more over 700°C was investigated. It became clear that the value for 100,000 hours was exceeded at 700°C and 100MPa calculated from the Larson-Miller parameter at C=20. And surface appearance of crept specimen was investigated in detail.